1. FIELD OF THE INVENTION
This invention relates to a partial oxidation process for disposing of biological sewage sludge without polluting the nation's environment.
2. DESCRIPTION OF THE PRIOR ART
The term "sewage" as used herein may be broadly defined as "water-carried wastes," particularly from municipal sanitary sewage lines containing body wastes (excreta), household wastes, community waste such as street washings, etc., and some industrial wastes. The total solids content of the sewage in municipal lines is usually about 500 parts per million (p.p.m.) or more. The solids in sewage are mostly animal or vegetable substances, "organic matter," i.e. fats, carbohydrates, and proteins. Some mineral or inorganic constituents present include sand and clay, as well as the common mineral salts found in the water supply. Sewage sludge may contain a number of pathogens known to be health hazards to humans. Almost any type of organism may be found in sewage including bacteria, protozoa, spores and cysts.
When sewage is introduced directly into streams, nuisance results from the putrefactive decomposition of the aforesaid organic matter. This biochemical decomposition produces substances which quickly consume the dissolved oxygen in the water, causing the death by suffocation to practically all of the higher animals such as fish. Objectionable gases may also be produced, and there is the possibility of disease if pathogenic bacteria is present. The consumption of oxygen by sewage is called the "biochemical oxygen demand" or "BOD." BOD tests denote the amount of oxygen consumed by bacterial activity under standard sampling and test conditions, i.e. Standard Methods American Public Health Association. On the other hand, the term "chemical oxygen demand" or "COD" denotes the total oxidizable material present in the waste liquid regardless of whether or not it is biodegradable. COD may be determined by ASTM Test Method D 1252-67. Although the BOD and COD of a sample of waste material are not comparable in all respects, generally a reduction in COD would indicate a reduction in BOD.
Typical Chemical Composition, Ultimate Analysis of Combustibles, and Analysis of Inert Materials In Primary Sewage Sludge are shown respectively in Tables I, II and III.
TABLE I ______________________________________ TYPICAL CHEMICAL COMPOSITION OF UNTREATED PRIMARY SEWAGE SLUDGE Item Range ______________________________________ Total dry solids (TS), wt % 0.5-8.0 Volatile solids (% of TS) 60-80 Grease and fats 6.0-30.0 (ether-soluble, % of TS) Protein (% of TS) 20-30 Nitrogen (% of TS) 1.5-6.0 Phosphorus (P.sub.2 O.sub.5, % of TS) 0.8-3.0 Potash (K.sub.2 O, % of TS) 0-1.0 Cellulose (% of TS) 8.0-15.0 Iron (not as sulfide) 2.0-4.0 Silica (SiO.sub.2, % of TS) 15.0-20.0 pH 5.0-8.0 Alkalinity (mg/L as CaCO.sub.3) 500-1500 Organic acids (mg/L as HA.sub.c) 200-2000 Higher Heating Value BtU/lb 4000-7000 ______________________________________
TABLE II ______________________________________ TYPICAL ULTIMATE ANALYSIS OF COMBUSTIBLES IN PRIMARY SEWAGE SLUDGE ELEMENT Wt. % ______________________________________ Carbon 54.60 Hydrogen 7.90 Nitrogen 4.50 Oxygen 32.00 Sulfur 1.00 ______________________________________
TABLE III ______________________________________ TYPICAL ANALYSIS OF INERT NON-COMBUSTIBLE MATERIALS IN PRIMARY SEWAGE SLUDGE ELEMENT mg/kg DRY SOLIDS ______________________________________ Total Sodium 1,515 Water Soluble Sodium (mg/l) 83 Total Potassium 800 Water Soluble Potassium (mg/l) 53 Arsenic 4.58 Beryllium 0.38 Cadmium 3.27 Chromium 244 Copper 289 Iron 150 Lead 147 Zinc 468 Nickel 63 Mercury 0.68 ______________________________________
The most common ways of disposing of sewage sludge in the U.S. in 1984 were as follows: land applications including filling and burying--about 60 wt. %, incineration--about 20 wt. %, and ocean dumping--about 7 wt. %. Thousands of tons of sewage sludge are dumped into the ocean annually. For example, about 1500 dry tons of sludge were dumped in the ocean every day in 1988. Ocean dumping of sewage sludge is one of the major environmental problems of our large East Coast cities. A Federal ban enacted in 1988 prevents ocean dumping after Dec. 31, 1991. Until then, dumping fees from $100-$200 per dry ton will be charged based on the dry tonnage of sludge dumped at the disposal site, for example, in the Atlantic about 106 miles east of Cape May.
The amounts involved, shortage of land and high transporation costs, place future limitations on landfilling as a means for disposing of sludge. Incinerators have poor emission characteristics, including soot and particulate matter, toxic gases e.g. dioxane, and foul odors. Air quality standards of federal and state environmental protection agencies while placing severe limitations on the use of incinerators can be readily met by the subject process.
To provide the sewage sludge feed in the subject process, raw municipal sanitary sewage is separated into sewage sludge and liquid. Any suitable way for separating solids and liquid may be used. For example, in coassigned U.S. Pat. No. 3,687,646, which is incorporated herein by reference, screening, gravity settling, filtration, centrifugation, expression pressing, and combinations thereof may be used. Heat treating sewage sludge is described in Wastewater Engineering Treatment/Disposal/Reuse, Metcalf and Eddy Inc., McGraw-Hill, 1979, pages 636-637. Coking sewage sludge is disclosed in coassigned U.S. Pat. No. 3,507,788. However, none of these references taken singly or in combination teach or suggest applicants' improved partial oxidation process for disposing of a pumpable slurry comprising coal or other fuel in admixture with an upgraded and agglomerated sewage sludge without contaminating the environment. Advantageously by the subject process, steam or hot water for use in the process, and valuable synthesis gas, reducing gas, or fuel gas are produced.